Thin, flexible plastic films, such as Polypropylene, polyethylene, polyvinyl chloride, Tedlar.RTM. (a registered trademark of E. I. DuPont ee Nemours & Co. for polyvinyl fluoride films), Mylar.RTM. (a registered trademark of E. I. DuPont de Nemours & Co. for polyester films), and the like, have been found to be particularly useful as moisture, vapor and thermal barriers for a variety of products such as aircraft insulation blankets. These properties, especially thermal reflectance, can be improved by providing a thin, reflective metallized layer, typically on one side of the plastic film.
Such films, however, require reinforcement to enable them to withstand manufacturing and subsequent use. Reinforcement is generally provided by bonding reinforcing fibers to the film.
One way of providing a reinforced film is to cast the molten film plastic onto a scrim or reinforcing fibers so that the reinforcing fibers are carried within the film itself. This method is disadvantageous, however, since it requires the manufacturer to acquire the equipment needed for casting and forming the various flexible plastic films and for metallizing if practical.
Another way of providing reinforced film is to select the desired pre-formed film or metallized film, and reinforce the film by adhesively bonding the reinforcing fibers or yarns to at least one side of the film. Petroleum and organic based solvent adhesives have been widely used in the past to bond such reinforcing fibers to the film because of the speed of cure which is possible with such adhesives. However, the use of petroleum and organic based solvent adhesives is declining due to stricter air quality control laws. Further, solvent based adhesives may corrode aluminum, thus limiting the use of such reinforced films in aviation or in other environments in which aluminum is used. While the use of many water based adhesives and hot melt adhesives will meet present air quality standards, and will not corrode aluminum, the use of such adhesives in an industrial process for reinforcing films requires the application of heat to rapidly dry water based adhesives, or melt and cure hot melt adhesives to rapidly bond the reinforcing fibers to the plastic film. However, the application of heat to cure such environmentally acceptable adhesives has caused other problems, including severe curling and puckering of the reinforced film. In reinforced metallized film, in particular, the reinforcing fibers are sometimes preferably bonded to the metallized side to protect the metallized side of the film from abrasion and other environmental hazards. However, in addition to curling and puckering, when such reinforced metallized films are subjected to heating in subsequent manufacturing steps, for example during ultrasonic sealing operations, the metallic layer can be pulled away from the film, breaking the seal and adversely affecting the thermal reflectivity and other properties of the metallized film.
These are significant problems. Reinforced film which curls or puckers is difficult to handle during subsequent manufacture into useful products, as well as being cosmetically unacceptable. The curl is often so severe, that a subsequent manufacturing step is required to relax the curl. Lengths of the curled film are placed in large ovens for days at a time until the film relaxes sufficiently to enable its use, thus increasing both the cost and time for producing reinforced film. Thus, the need exists for a flexible, flat reinforced film which is thermally stable in all operating environments.